Various methods have been proposed for pyrolyzing the hydrocarbonaceous fraction of certain naturally occurring materials, such as oil shale, coal, tar sands, diatomaceous earth, etc. For example, fluidized beds, partially fluidized beds, packed beds, and mechanical mixers have been suggested as a means for rapidly heating the hydrocarbonaceous materials to a pyrolyzing temperature. The use of a solid particulate heat-transfer material physically admixed with the hydrocarbonaceous material is one preferred method for raising the hydrocarbonaceous material to the desired temperature. Although advantageous, the use of a solid heat-transfer material requires that a practical commercial process be able to handle larger amounts of solids than otherwise would be required. This requirement, along with a need for rapid mixing, sufficient residence times for recovery of the pyrolysis products, and the ability to handle a diversity of particle sizes places stringent limitations on existing processes. For example, fully fluidized beds offer the advantage of rapid mixing of the solids in the bed to achieve thermal equilibrium between the hydrocarbonaceous material and the heat-transfer solid. However, the rapid top-to-bottom mixing characteristic of fluidized beds also makes it difficult to control the residence time of solids passing through the bed. It is desirable to achieve substantial plug flow conditions in the bed by limiting gross overall mixing of the solids. Such a condition is more characteristic of packed beds than of fluidized beds. However, packed beds have very poor mixing of the solids and tend to form "hot" and "cold" spots in the bed dependent upon the distribution of the heat-transfer solid. Thus, if a packed bed is used to pyrolyze the solid, some means for premixing the solids must be employed prior to introducing them into the packed bed.